Scanning Probe Microscopy (SPM) techniques such as Atomic Force Microscopy (AFM) and Near-Field Scanning Optical Microscopy (NSOM) are very useful techniques for the study of surfaces. Acquired SPM images provide surface properties (e.g., topography or roughness) on a nanometer scale. As a result, SPM instruments are now used routinely for testing in industries where small dimensions are involved. A typical example is the semiconductor industry where AFM is used to evaluate the surface roughness and topography following a film deposition or a chemical mechanical polishing (CMP) step. Another important application of AFM in the semiconductor industry is the measurement of deep trenches with a very high aspect ratio (height to width ratio of greater than 10.0). The trenches are used for fabricating capacitors, which form the memory cache of a chip.
Most SPM instruments record the surface features using a sharp tip that is fabricated at the end of a cantilever. In general, the surface image is the convolution product between the tip and the features of the surface (the data one wishes to acquire). As such, the “sharper” the tip, the more closely it will approximate a spatial delta function, which will result in a more accurate imaging of the surface.
The actual radius of curvature of the tip's end that should be considered as affecting imaging depends on the type of interaction between the tip and the surface. Many tip-surface interaction decay as 1/rn where r is the distance, and n is a number between 2 to 6. Specifically, and as an example, for a Lennard Jones pair potential between a surface and a sphere, n in the above equation is equal to 3. This is considered a long-range interaction, which suggests that a high aspect ratio tip is required to enhance the instrument's resolution. In other words, since the interaction has a long range, parts of the tip body that are distant from its end (but not necessary from the surface, in the case of a tip with large radius of curvature) also contribute to the measured interaction. A tip with a high length over width aspect ratio minimizes these contributions. In addition, a high aspect ratio tip is required for the inspection of deep trenches.
Conventional AFM tips are fabricated by anisotropic etching of silicon, Si. These tips are formed at the end of a silicon cantilever and have the shape of a pyramid with triangular sides defined by Si (111) surfaces. Since these tips are fabricated by batch silicon processing techniques (also referred to as micromachining), they are sold as full wafers and their cost is in the dollar range. The drawback of these prior art tips is their poor imaging resolution due to their low aspect ratio and large radius of curvature of about 5-30 nm (best case). An example of a pyramidal silicon tip is shown in FIG. 1A. FIG. 1B is a pictorial representation of how an AFM tip is used to map the surface topography. In this drawing, reference numeral 103 represents the analyzed surface, reference numeral 101 represents the Si tip, and reference numeral 102 represents the cantilever. A typical silicon wafer with micromachined AFM probes is illustrated in FIG. 2. In this drawing, reference numeral 201 represents the wafer with micromachined AFM probes, reference numeral 203 denotes the Si tip and reference numeral 202 denotes the cantilever.
To enhance the resolution, a carbon nanotube (CNT) can be glued to the end of a conventional Si tip (See, for example, J Martinez et al., “Length control and sharpening of atomic force microscope carbon nanotube tips assisted by an electron beam”, Nanotechnology 16 (2005) 2493-2496). The use of a CNT provides not only a small radius of curvature, but also a high aspect ratio probe. However, due to their manual fabrication the cost of a probe with a mounted CNT tip is roughly 1000× of a conventional micromachined AFM tip.
In view of the foregoing, there is a need for a batch fabrication process that yields CNT like AFM tips (i.e., with a radius of curvature of a few nanometers and an aspect ratio of 1:100) at the cost of a conventional micromachined AFM tip.